1. Technical Field of the Invention
This invention relates to a porous gas diffusion electrode and a method of preparing the same and more particularly to a porous gas diffusion electrode provided with end portion seals constituting an electricity generating element which can be used for air-metal cells and fuel cells, be easily laminated, and used with high reliability and also to a method of producing the same.
2. Technical Background of the Invention and its Problems
Air-metal type cells or hydrogen-air (oxygen) fuel cells are required to have electrodes for ionization of gas as their electromotive mechanisms. The gas diffusion electrode, in which a gas having no electroconductivity participates continuously in electromotive reaction, employs a porous plate substrate and is so constituted that one surface of the substrate is brought into contact with an electrolyte, and the gas flowing into the substrate from the other surface is permitted to participate in the electromotive reaction within the substrate. In general, for feeding a reaction gas, reaction gas feed channels are provided on the substrate itself, or the substrate is used so as to be contacted with another element having reaction gas feed channels.
In these cell systems, each unit cell outputs a voltage of at most 0.5 to 1.5 V, and hence a considerable number of unit cells are required to be laminated and provided for practical use. During lamination, unit cells are laminated in such a manner that the reaction gas feed channels provided on one substrate itself or provided in contact with the substrate during use under lamination are in a direction to cross over other gas feed channels provided above or below. Accordingly, it is necessary to treat the end portions on the side faces parallel to the respective gas feed channels so that one reaction gas, for example, hydrogen gas, flowing through one reaction gas channel and another gas, for example, air, flowing through another gas feed channel above or under said channel will not become mixed with each other. At the same time, it is also necessary to treat the end portions so as to maintain electrical insulation even when compressed mechanically strongly by lamination of a large number of units.
In order to prevent mixing of different gases at the end portions of the substrate parallel to gas feed channels and to ensure electrical insulation at the end portions, a method in which the end portions are coated with synthetic resins and a method in which they are impregnated with a dispersion of a tetrafluoroethylene copolymer have been proposed. However, none of these methods has been successful in obtaining satisfactory results.